Abstract: To address the problems of high water cut and low recovery degree in the deep low-permeability heavy oil reservoir of the Guan 128 block in Dagang Oilfield, a study on the mechanisms of enhanced oil recovery by viscosity reducer flooding and the optimization of development strategies is carried out. By integrating micro-scale seepage experiments with reservoir numerical simulation, the microscopic flow behavior of viscosity reducer flooding is systematically investigated, and the optimal development parameters are determined based on the characteristics of the target reservoir. The results show that, after the injection of viscosity reducers, in situ emulsification of heavy oil occurs within the pore space, forming O/W emulsions, which enhance microscopic oil displacement efficiency and enlarge the sweep volume. During the later stage of displacement, the emulsion droplet size gradually decreases, the influence of the Jamin effect weakens, and the emulsion can migrate rapidly through pore throats, thereby promoting residual oil mobilization. The optimization results indicate that, under the huff-and-puff mode, the optimal soaking time is 4 days, the optimal cyclic injection volume is 1,200 m³ (viscosity reducer solution), and the optimal mass concentration is 2%. For the target block, a single-slug injection strategy with a viscosity reducer concentration of 0.5% and a slug size of 0.2 PV can improve the recovery factor by approximately 5 percentage points. The study demonstrates that viscosity reducer flooding effectively enhances the development performance of deep low-permeability heavy oil reservoirs through the synergistic effects of in situ emulsification and flow regulation. The findings provide theoretical support and technical guidance for the mechanism study and development scheme design of viscosity reducer flooding in similar reservoirs.